A current limitation mode is often supported in many modern power switches. In current limitation mode, a limit may be imposed on current (ID) flowing through a power switch. The value of the current limitation may be dependant on a voltage drop across the power switch. For example, if the power switch is a DMOS transistor, then the value of the current limitation may be based on a drain-source voltage (VDS) of the transistor.
FIG. 1a illustrates an example of a two-level current limitation with a curve 105 displaying a current flowing through a power switch. A two-level current limitation may include asserting a high current limitation ID HIGH if VDS is less than a threshold (VDSX) and a low current limitation ID LOW if VDS is greater than the threshold. Therefore, if VDS is less than VDSX, then the high current limitation ID HIGH would be asserted and if VDS is greater than VDSX, then the low current limitation ID LOW would be asserted.
However, in a fault condition, such as a short circuit at an output of the power switch in combination with typical impedances of cabling/wiring used to connect the power switch to a battery and/or a load, for example, current oscillation may occur if VDS is slightly larger than VDSX. If VDS is slightly higher than VDSX, then ideally the low current limitation ID LOW may be used to regulate the current flowing through the power switch. Then, in a fault situation wherein the current ID begins to rise, the voltage VDS may drop below the threshold VDSX due to the impedance of the cabling/wiring. The change in the voltage VDS may be expressed as:
                    V        coil            ⁡              (        t        )              =          L      *                        ⅆ                      i            ⁡                          (              t              )                                                ⅆ          t                      ,where Vcoil(t) is the voltage VDS, i(t) is a time varying current, and L is the inductance of the cable/wiring.
As the voltage VDS drops below the threshold VDSX, then the high current limitation ID HIGH may be used to regulate the current flowing through the power switch instead of the low current limitation ID LOW. Then, when the current flowing through the power switch achieves the high current limitation ID HIGH, a voltage drop over the cabling/wiring may decrease, thereby causing the voltage VDS to increase. If the voltage VDS increases over the threshold VDSX, then once again, the low current limitation ID LOW may be used to regulate the current flowing through the power switch. This alternating behavior is referred to as current oscillation. The current oscillation may continue until a control signal of the power switch changes or the power switch shuts down due to overheating.
FIG. 1b illustrates a data plot 150. The data plot 150 displays a first trace 155 displaying a power switch control signal (labeled “IN”), a second trace 160 displaying a voltage produced by a battery coupled to the power switch (labeled “Vbat”), a third trace 165 displaying an output voltage (labeled “Vout”), and a fourth trace 170 displaying a current flowing through the power switch (labeled “ID”). The drain-source voltage VDS may be related to the output voltage Vout, for example, in a relationship expressible as VDS=Vbat−Vout. Therefore, when the output voltage Vout is increasing, the voltage VDS may be decreasing, and vice versa.
As the output voltage Vout increases (highlight 175), the voltage VDS may be decreasing. When the voltage VDS drops below the threshold VDSX, the high current limitation ID HIGH may be used to regulate the current flowing through the power switch instead of the low current limitation ID LOW. The change in the current limitation may allow for the current ID to increase from a low value to a high value (highlight 177). As the current ID increases and reaches the high current limitation ID HIGH, the voltage VDS may also increase and may once again exceed the threshold VDSX (shown as decreasing output voltage Vout and highlight 179 on the third trace 165). When the voltage VDS increases above the threshold VDSX, the low current limitation ID LOW may be used to regulate the current flowing through the power switch instead of the high current limitation ID HIGH and the current ID may decrease (highlight 181).
As shown in FIG. 1b, the current oscillations may repeat. The current oscillation may stop if the switch control signal changes value or the power switch overheats and shuts down to help prevent damage to the power switch. If the power switch shuts down to prevent damage from overheating, once it is back on, the current oscillation may continue until the switch control signal changes or the power switch overheats once again.